1. Field of the Invention
The present invention relates to solid fuel delivery systems, and more particularly to solid fuel nozzle tips for issuing solid fuel into boilers.
2. Description of Related Art
A variety of systems and devices are known for delivering solid fuel for combustion in a boiler. Many such devices are directed to nozzles for delivering solid coal particles to coal fired boilers or furnaces, for example. Coal powered plants require an efficient means of supplying coal as fuel to produce heat power. Raw coal is typically pulverized in a coal pulverizer or mill to produce small coal particles or coal dust. The pulverized coal must then be delivered to a furnace or burner where it can be used for combustion. This is typically done with a coal piping system that utilizes air flows to transport pulverized coal particles from the mill or pulverizer to a nozzle where coal particles are injected into the coal burner or furnace.
A great deal of effort has been made to design coal tip nozzles capable of providing controlled, evenly distributed streams of coal and air. Non-uniform particle distribution causes various technical problems for operation and maintenance of coal systems. If poor particle distribution extends into the combustion zone, localized imbalances in the fuel/air mixture can cause inefficient combustion and elevated emissions of NOx, CO, and other pollutants. It can also cause elevated levels of unburned carbon in the fly ash, which will lower combustion efficiency.
In order to improve flow and velocity distribution, known coal tip nozzles have incorporated flow vanes, splitter plates, multiple shrouds, and the like to provide desirable flow characteristics. Typical coal tip nozzles are constructed with the shrouds, vanes, and splitter plates all welded together into a single solid piece. However, the heating on typical coal tip nozzles is uneven. Uneven heating results from temperature gradients across the nozzle tip, ranging from the high temperature at the outlet, which is exposed to flame temperature within the boiler or furnace, to the relatively cool flow of air and coal particles entering the nozzle tip at the inlet. All of the components experience different amounts of heating and there is typically an appreciable difference experienced by the inner and outer shrouds of typical designs. The differential thermal expansion in typical designs results in internal stresses which can lead to failure and limited service life.
One attempt to address the thermal expansion gradients in typical coal tip nozzles has been to recess vanes or support means mounted between inner and outer shrouds back from the outlet. Such a configuration is shown in U.S. Pat. No. 6,089,171 to Fong et al. This approach, however, is still relatively restrictive to thermal expansion and contraction of inner and outer nozzle components. In addition, the recessed vanes have reduced ability to channel flow through the nozzle.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for solid fuel tip nozzles that allow for improved accommodation of thermal expansion. There also remains a need in the art for such devices that are easy to make and use. The present invention provides a solution for these problems.